On a site for working radioactive ores, it is common practice from time to time to measure abundance by measuring the radioactivity of the extracted ore. Such measurement is used mainly for sorting ore into grades of different abundances, and in order to eliminate ore which is considered as being sterile or too poor, while also making it possible to obtain an indication about the production from a site over a given period of time by summing all of the measurements performed. Overall, such measurements serve to optimize the working of a quarry.
In general, prior measurement devices comprise a radiation-measuring gate of such a size as to enable dumper trucks or other ore-conveying machinery to pass through the gate, with the gate including one or more top cross-members having various radioactivity detectors mounted thereon. The detectors may be Geiger-Muller tubes or monocrystal scintillation meters suitable for detecting the gamma rays given off by the load and related to an appropriate counter. After a predetermined exposure time, e.g. a few seconds, the counter gives an indication on the abundance of radioactive elements in the ore contained in the bin of the machine.
However, radioactivity measuring gates suffer from several drawbacks.
Firstly, the measurement of radioactivity as obtained is extremely inaccurate. For a mass of several tens of tons of ore in the bin of a truck, only those gamma rays coming from the top layer of the ore (over a thickness of about 60 cm) escape from the mass and reach the detectors at the top of the gate. It will be understood that in the specific case of the mass of ore in the bin being non-uniform, measurement errors can be considerable.
In addition, calibrating any type of device for measuring radioactivity requires sampling campaigns that are very burdensome and expensive. For example, with a radioactivity measuring gate, it is necessary to process about 30 bin loads each weighing 25 to 30 tons during one such campaign, thereby obtaining a statistical curve of radioactivity as a function of abundance in the ore which is good enough for the gate to be useful in practice.
Finally, radioactivity measuring gates are expensive, and as a result it is not uncommon for there to be too few gates provided in any one quarry. In practice, this gives rise to lines of waiting trucks or other machines queuing up to go through the gate, thereby reducing productivity.
Attempts have long been made in vain to fit the shovels or buckets of ore-extracting machines with devices for measuring the radioactivity of a load of ore contained therein. However these attempts have given rise to failures. More precisely, currently available gamma ray detectors are all relatively fragile in structure and they do not withstand the rough working conditions to which a hydraulically operated shovel or other comparable machine is subjected in operation.
Consequently, research directed towards this type of measurement has now been abandoned.
By going against this trend in the art, the Applicant has nevertheless sought to develop a machine-mounted measurement device which is effective in the difficult environment of a quarry. This research has lead to the present invention.